The coronavirus has thrown yet another curveball, possibly producing a new illness in children, who have been thought to be largely spared from the ravages of the respiratory disease.

It’s called pediatric multisystem inflammatory syndrome associated with COVID-19, and 64 children have been hospitalized throughout New York with the illness, according to an advisory issued Wednesday by the state’s health department.

“Thankfully most children with COVID-19 only experience mild symptoms, but in some, a dangerous inflammatory syndrome can develop,” said New York Health Commissioner Dr. Howard Zucker. “While we continue to reduce cases through social distancing, discoveries like this remind us we are still in the middle of our response to this deadly pandemic.”

Other cases have been reported in California, Mississippi and Louisiana, according to The New York Times.

The multisystem inflammatory disease can occur days to weeks after acute coronavirus illness, and it bears features of toxic shock syndrome and Kawasaki disease, a rare illness of unknown origin that causes blood vessels to become inflamed.

It can cause persistent fever, abdominal symptoms, rash and cardiovascular symptoms, as well as heart-related or blood vessel-related shock requiring intensive care.

The majority of the patients tested positive for COVID-19 either through molecular testing for SARS-CoV-2 or through blood testing.

“Most children are not affected by the coronavirus, and reports of children who have become seriously ill remain rare and unusual cases,” said Dr. Sean O’Leary, a member of the American Academy of Pediatrics Committee on Infectious Diseases.

While some children who have been reported to have this condition tested positive for the coronavirus, he noted that others have not.

“The condition has the features of fever and evidence from lab tests of significant inflammation in the body. Some of these children are very ill with inflammation in their intestines, their heart or other organs,” Dr. O’Leary said, describing the syndrome as being distinct from other rare childhood conditions. “Scientists around the world are working hard to understand this syndrome and how best to treat it.”

A possible link between COVID-19 and the inflammatory disease has been reported in the United Kingdom, where a small rise in the number of critically ill children exhibiting symptoms was reported late last month.

More than 23,000 children in the U.S. have tested positive for COVID-19 as of April 30, according to data from the American Academy of Pediatrics. Most children with COVID-19 are either asymptomatic or experience mild infections. While children can become very sick and need hospitalization, it is rare.

“Reports about these new pediatric COVID-19-associated cases are very preliminary. We will learn more as research and details about ongoing and future cases are compiled by the medical community and published for more complete review,” Dr. Mark Hicar, a pediatric infectious disease physician and assistant professor at University at Buffalo, wrote in The Conversation, an online academic journal.

He cautioned against jumping to conclusions about the illness and comparing it to Kawasaki disease, pointing out some differences.

“That being said, some children with Kawasaki disease will test positive for the new coronavirus. In North America, there are estimated to be 5,000 to 6,000 annual cases of Kawasaki disease. Overlaying more than 1 million cases of COVID-19 throughout the United States, there will be some overlap,” Dr. Hicar said.

Parents of children with suspected cases of the inflammatory disease are recommended to consult their pediatricians. The American Academy of Pediatrics says parents should not be afraid to bring their children to see a pediatrician for sick or well-child visits.

Hospitals in New York are required to immediately report cases of pediatric multisystem inflammatory syndrome associated with COVID-19 in patients younger than 21 years old.